Since the nitride semiconductor material is known to have a sufficiently large band gap and besides its inter-band transition is of direct transition type, many investigations for utilizing the nitride semiconductor material in the short wavelength light emission device are underway. Furthermore, as its saturation drift velocity of electrons is high and the two-dimensional carrier gas is available in their hetero-junction, the nitride semiconductor material is also regarded to be highly applicable to the electron device.
The nitride semiconductor layer to constitute these devices can be obtained by epitaxial growth on a base substrate with the vapor phase deposition method such as metal-organic vapor phase epitaxy (MOVPE) method, molecular beam epitaxy (MBE) method or hydride vapor phase epitaxy (HVPE) method. However, there is not any base substrate that has a lattice constant matching with that of this nitride semiconductor layer, and, therefore, a growth layer of good quality is hard to acquire and the nitride semiconductor layer obtained tends to contain numerous crystal defects. Because these crystal defects are a very factor to hinder the improvement of device performance, variety of approaches to decrease the crystal defects within the nitride semiconductor layer have been so far examined.
As one of the methods to obtain group III element nitride based crystals containing a relatively small number of crystal defects, there is known a method wherein a low temperature deposition buffer layer is formed on a substrate of a different material such as sapphire and thereon an epitaxial growth layer is formed. In the crystal growth method using a low temperature deposition buffer layer, deposition of AlN or GaN onto a sapphire substrate or such is first applied around 500° C. to form an amorphous film or a continuous film containing, in part, poly-crystals. By heating this deposition up to about 1000° C., a part of the deposition is evaporated away and the remains are converted into crystals to form crystal nuclei of high density. Application of those as nuclei for crystal growth leads to GaN layer of relatively high crystalline quality. Nevertheless, even using the method comprising the step of forming the low temperature deposition buffer layer, it still contains a considerable number of crystal defects such as threading dislocations and vacant pipes, and, thus, its crystalline quality is insufficient to provide such high performance devices as currently required.
Alternatively, another technique in which a GaN substrate is used as a substrate for crystal growth and thereon a semiconductor multi-layered film for constructing a device section is formed has been extensively studied. Such a GaN substrate for crystal growth is referred to as a self-supporting GaN substrate, hereinafter. Among techniques to prepare a self-supporting GaN substrate, the ELO (Epitaxial Lateral Overgrowth) technique is widely known. The ELO is a technique in which a mask layer having stripe openings is formed on a base substrate and, the lateral growth is initiated from the openings to attain a GaN layer with a few dislocations. In Japanese Patent Application Laid-open No. 251253/1999, it is proposed that a GaN layer is formed on a sapphire substrate using this ELO technique, and thereafter the sapphire substrate is removed by etching or such to prepare a self-supporting GaN substrate.
Meanwhile, the FIELO (Facet-Initiated Epitaxial Lateral Overgrowth) technique (A. Usui et al., Jpn. J. Appl. Phys., Vol. 36 (1997) pp. L899–L902) has been developed as one of the techniques progressed from the ELO technique. This technique shares common ground with the ELO in the point of carrying out the selective growth using a silicon oxide mask, but differs from the ELO in the point of forming facets, thereat, in mask opening sections. Formation of facets changes the propagation direction of dislocations and, thus, reduces the number of threading dislocations that reach the top of the epitaxial growth layer. With this method, a self-supporting GaN substrate of high quality having a relatively small number of crystal defects can be obtained by the process where a thick GaN layer is grown upon abase substrate of, for instance, sapphire, and subsequently the base substrate is removed from that.